1. Field of the Invention
The present invention relates to a ring oscillator, and more particularly to an oscillator for generating an operation clock signal used in microcomputers and the like and a semiconductor integrated circuit and an electronic device including the same.
2. Description of the Related Art
In microcomputers and the like, a desired operation frequency is generally obtained by lowering the frequency of an original clock signal inputted from an external quartz oscillator with an internal frequency divider. In view of the present situation that microcomputers have become inexpensive, however, the above way of providing an external quartz oscillator to obtain an operation clock signal for a microcomputer has no cost merit. Also, an input buffer section of a microcomputer that receives an original clock signal from an external quartz oscillator is still required to operate at high speed, and this has become a cause of blocking reduction in the power consumption of microcomputers. Therefore, from the standpoints of low cost and low power consumption, microcomputers and the like preferably incorporate therein an independent oscillator that generates a comparatively low-speed operation clock signal. As such an independent oscillator, a differential ring oscillator composed of a plurality of reverse delay circuits connected in a loop may be used.
While a differential ring oscillator is less susceptive to variations in power supply voltage and ambient temperature and the like and therefore attains sufficiently high oscillation frequency precision, it is necessary to provide a bias circuit for adjusting the bias of each reverse delay circuit. In consideration of the cost, an oscillator incorporated in a microcomputer and the like is preferably simpler in configuration. Also, for improvement of the yield, an oscillator is preferably provided with a mechanism for compensating fabrication variations.